Kyoungsoo Kim

Theoretical Investigation of First-order and Second-order Polarization-mode Dispersion Tolerance on Various Modulation Formats in 40 Gb/s Transmission Systems with FEC Coding

We investigated the polarization-mode dispersion (PMD) tolerance for 40Gb/s non-return to zero (NRZ), duobinary NRZ, return to zero (RZ), carrier-suppressed RZ (CS-RZ), and duobinary-carrier-suppressed RZ (DCS-RZ) modulation formats with a forward error correction (FEC) coding. The power penalty has been calculated as a measure of the system performance due to PMD. After comparison of the PMD tolerance of various modulation formats, our results suggest that RZ signals have the best tolerance against the effect of first-order PMD only. The duobinary NRZ modulation format is most resilient to PMD when both first- and second-order PMD are considered. However, the duobinary NRZ modulation format is the most sensitive to the incident angle of the input signal to a fiber axis in the presence of first- and second-order PMD, leading to incident angle-dependent power penalty. The coding gain by FEC can cope with the power penalties induced by first- and second-order PMD up to a DGD value of 16ps.

Performance Limitations of Subcarrier Multiplexed WDM Signal Transmissions Using QAM Modulation

We theoretically investigate the performance limitations of subcarrier multiplexed (SCM) wavelength-division-multiplexing (WDM) systems using optical double-sideband (DSB) modulated, 16-quadrature amplitude-modulated (QAM) signals. The performance limitations are investigated using crosstalk power and SCM channel spacing for various transmission conditions, including impairment factors such as dispersion and fiber nonlinearities for a single wavelength channel first. The effects of WDM channel spacing on SCM systems with multiwavelength channels are also evaluated via the calculated bit error rate (BER) performance, based on the performance limitations found in the single-wavelength simulation. This enables us to provide guidelines for the design of SCM/WDM systems for fiber-to-the-home (FTTH) network in WDM-passive optical network (PON) architecture, based on the performance limitations.

Comparison of Transmission Performance of 40-Gb/s Optical Duobinary and DCS-RZ Signals Using 10- and 40-Gb/s

We investigated a cost-effective method using 10-Gb/s LiNbO3 Mach-Zehnder modulators (MZMs) for transmitting 40-Gb/s duobinary nonreturn-to-zero (NRZ) and duobinary carrier-suppressed return-to-zero (DCS-RZ) signals. The transmission performance of the signals was investigated. The optimum bandwidths of low-pass filters in transmitters using 10- and 40-Gb/s LiNbO3 MZMs were also investigated. The calculated transmission performance of the transmitter using 10-Gb/s LiNbO3 MZMs was compared to that using 40-Gb/s LiNbO3 MZMs. The effect of the bias offset in the transmitters was investigated. Compared to duobinary-NRZ and DCS-RZ signals generated by a transmitter using 40-Gb/s LiNbO3 MZMs, those generated by using 10-Gb/s LiNbO3 MZMs can be transmitted with 1-dB power penalty over 480-km transmission

\hbox{LiNbO}_{3}

Forward-error correction (FEC) coding is theoretically investigated to improve bit-error-rate (BER) performance in a 10-Gb/s optical transmission system using randomly irregular low-density parity-check (LDPC) codes, regular LDPC codes, and the Reed-Solomon (RS) (255,239) code as a comparison. The irregular LDPC codes has different row-weight variances of a parity-check matrix from 10.9 to 18.8 and a row-weight mean of 60. Simulation is carried out under various conditions including the impairment factors such as dispersion, polarization-mode dispersion (PMD), and fiber nonlinearities. Results suggest that the irregular LDPC code with a low row-weight variance (=10.9) generally has better performance for the most impairment factors except for the factor of dispersion. On the other hand, for the factor of dispersion the irregular LDPC code performs better with a high row-weight variance (=18.8). A specific LDPC code can overcome the impairment limits in a deployed link.

Mach–Zehnder Modulators

We investigated the combined effect of fiber chromatic dispersion and self-phase modulation (SPM) in multi-channel subcarrier multiplexed (SCM) optical transmission systems. We theoretically analyzed the transmission characteristics of the SCM signals with the effect of SPM and chromatic dispersion in a single-mode optical fiber by numerical simulations based on the nonlinear Schrodinger equation. The numerical simulation results revealed that the effect of fiber dispersion and SPM could occur independently between subcarrier channels in two-channel SCM systems for small optical modulation index (OMI) and large channel spacing. However, for large OMI, small channel spacing, and large fiber launching power, we found a performance degradation of the two-channel system compared to that of a single-channel system. These parameters are therefore important for the optimization of multi-channel SCM systems applicable to radio over fiber networks.

BER performance due to irregularity of row-weight distribution of the parity-check matrix in irregular LDPC codes for 10-gb/s optical signals

We investigated how the polarization-mode dispersion (PMD) tolerance was degraded by semiconductor optical amplifier (SOA)-induced chirp for the 10 Gb/s nonreturn-to-zero (NRZ), duobinary NRZ, return-to-zero (RZ), and carrier-suppressed RZ (CS-RZ) modulation formats. The power penalty was calculated as a measure of the system performance due to PMD for a given SOA-induced chirp. Considering only first-order PMD, all modulation formats have a similar PMD tolerance regardless of SOA-induced chirp. On the other hand, when both first- and second-order PMD are considered, the PMD tolerance of all modulation formats with the exception of the CS-RZ modulation format are degraded by SOA-induced chirp. Among all modulation formats considered here, the NRZ modulation format has the PMD tolerance with the highest sensitivity to SOA-induced chirp. When the peak-to-peak chirp induced by SOAs is

Effect of Fiber Dispersion and Self-phase Modulation in Multi-channel Subcarrier Multiplexed Optical Signal Transmission

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PMD Tolerance of 10 Gbps Modulated Signals due to SOA-Induced Chirp in SOA Booster Amplifiers

, its PMD tolerance is degraded up to 4 dB for a differential group delay (DGD) of 50 ps. However, the PMD tolerance of the CS-RZ modulation format is largely unaffected by SOA-induced chirp.

SPM-induced power gain in optical subcarrier-multiplexed transmission systems

We theoretically investigated the SPM-induced power gain for optical subcarrier multiplexed (SCM) signals under the combined effects of fiber chromatic dispersion, self-phase modulation (SPM), and cross-phase modulation (XPM) in subcarrier multiplexed (SCM) optical transmission systems.

Generalized Analysis on the Combined Effect of SPM and Fiber Chromatic Dispersion on Subcarrier Multiplexed Optical Transmission Systems for RoF Applications

We investigate theoretically the combined effect of fiber chromatic dispersion and self-phase modulation (SPM) on multi-channel subcarrier multiplexed (SCM) optical transmission systems in terms of the detected RF carrier power and SPM-induced power gain after transmission over single-mode fiber (SMF) links. According to the calculated power gain due to the SPM effect at the transmission distance of P3dB using the detected radio-frequency (RF) carrier power after photo-detection, the power gain is significantly degraded with large optical modulation index (OMI), small SCM channel spacing, and large fiber launching power because of the increased interaction between subcarrier channels. The nonlinear phase shift due to linear and nonlinear fiber characteristics is investigated to explain these results in detail. The numerical simulation results show that the OMI per SCM channel has to be smaller than 10 % for the fiber launching power of 10 dBm to guarantee prevention of SPM-induced power gain degradation below 0.5 dB for the SCM system with the channel spacing of 100 MHz. This result is expected to be utilized for the optical transmission systems using the SCM technology in future radio-over-fiber (RoF) networks.